If you download a custom scenario that requires a dedicated .eqp file it usually means that you need to look for a file or folder that you need to put into Graphics folder in the main game directory -in the root directory to which the game is installed. Normally a custom scenario comes with either a file or custom folder bearing the scenario name and it needs to be unpacked into the Graphics folder. You need to be careful not to overwrite the stock game .eqp file so sometimes it is wise to use the AltGraphics folder instead, but I know of the scenarios that don't accept this shortcut and work only if their custom .eqp file is put into main Graphics folder.